1. Field of the Invention
The present invention relates to a photovoltaic cell which is excellent in photovoltaic transduction efficiency (photovoltaic transfer efficiency).
More particularly, the present invention relates to a photovoltaic cell which realizes high electyron moving velocity, ensures prolonged stability of electrolyte layer and exhibits high photovoltaic transduction efficiency. Still more particularly, the present invention relates to a thin photovoltaic cell for solar cell, or thin flexible photovoltaic cell for solar cell, exhibiting high energy transduction efficiency.
2. Description of Related Art
Photovoltaic transducers are a material from which light energy is continuously taken out as electric energy and a material which converts light energy to electric energy by the utilization of an electrochemical reaction between electrodes. When a photovoltaic transducer material is irradiated with light, electrons are generated from one electrode. The electrons move to a counter electrode, and the electrons having reached the counter electrode return by migrating as ions through an electrolyte to the one elect-rode. This energy conversion is continuously carried out, so that it is utilized in, for example, a solar cell.
The common solar cell is produced by first forming an electrode on a support such as a glass plate coated with a transparent conductive film, subsequently forming a semiconductor film having a photosensitizer adsorbed thereon on a surface of the electrode, thereafter providing a counter electrode comprising a support such as a glass plate coated with another transparent conductive film, sealing an electrolyte between the counter electrode and the semiconductor film, and finally sealing the side faces with a resin or the like.
When the above semiconductor film is irradiated with sunlight, the photosensitizer adsorbed on the semiconductor absorbs visible-region rays to thereby excite itself. Electrons generated by this excitation move to the semiconductor, next to the transparent conductive glass electrode, and further to the counter electrode across a lead connecting the two electrodes to each other. The electrons having reached the counter electrode reduce the oxidation-reduction system in the electrolyte. On the other hand, the photosensitizer having caused electrons to move to the semiconductor is in oxidized form. This oxidized form is reduced by the oxidation-reduction system of the electrolyte to thereby return to the original form. In this manner, electrons continuously flow. Therefore, functioning as the solar cell can be realized.
The electrolyte to be sealed between the electrodes is dissolved in a solvent, selected according to the type of the electrolyte, to thereby obtain an electrolytic solution. The electrolytic solution is sealed in a cavity created by sealing the sides of the photovoltaic cell with, for example, a resin.
The above solvent can be selected from among, for example, water, alcohols, oligoesters, carbonates such as propione carbonate, phosphoric acid esters, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, N-vinylpyrrolidone, sulfur compounds such as sulfolane 66, ethylene carbonate and acetonitrile.
However, when the electrolyte is used in the form of such an electrolytic solution, the solar cell may suffer from, during a long-term service, alteration of solvent molecules, decomposition of solvent molecules, vaporization of low-boiling-point solvent, leakage of electrolytic solution (solvent and/or electrolyte) from sealed parts, etc. with the result that the performance, such as photovoltaic transduction efficiency, thereof is deteriorated. That is, the use of the electrolyte in the form of an electrolytic solution has a drawback in that the long-term stability is poor.
Moreover, depending on the type of electrolyte used in the electrolytic solution, hygroscopicity is exhibited to thereby absorb water, and it may occur that the water causes the electrolyte and the photosensitizer to decompose to result in performance deterioration.
The photovoltaic transduction efficiency of the current solar cell is not always satisfactory and there are limitations in the application thereof. Therefore, there is a demand for further enhancement of light utilization ratio.
It is an object of the present invention to provide a photovoltaic cell which is excellent in long-term stability, ensures high light utilization ratio and exhibits high photovoltaic transduction efficiency.